The present invention concerns a process for the local removal of a coating, also called-xe2x80x9ccoveringxe2x80x9d consisting of at least one metallic layer and/or at least one metal-oxide layer, applied on a translucent or transparent substrate made of natural or synthetic glass or of transparent plastic.
By xe2x80x9ccoveringxe2x80x9d is understood any coating composed of one or several layers lending particular optical, electromagnetic or thermal qualities to the substrate.
Such coatings are often applied onto glass, for example for wind screens, often in one or several layers which are so thin that they are translucent or transparent.
Said coatings are usually vacuum applied and over the entire surface of the material. For many applications, the material must be stripped in particular spots, for example on the contour.
This local removal is necessary, for example to recover the original qualities of the material in view of a better adhesion if the glass is to be glued, in view of an electric insulation if the coating is conductive, in view of recovering the initial transparency in case of a non-transparent coating etc.
At present, the local removal of a coating is done by means of chemical or mechanical methods.
However, these techniques imply chemical pollution, and they can attack the surface or even damage the material itself.
Said removal of the coating can be avoided by applying the coating only to the desired spots by means of a masking technique.
However, it is not always possible to use such a technique, among others in the case of masks having a complex shape or representing interrupted and enclosed parts.
Moreover, the transitions between the coated parts and the non-coated parts are often of bad quality.
The cost of the masks is often prohibitive, especially if they need to be replaced for each different localisation of the non-coated parts.
The invention aims to provide a process for the local removal of a coating, applied on a translucent or transparent substrate, which avoids said disadvantages and which, as a result, is economical, does not include any risk of damaging the substrate, allows for a localisation with great precision and a transition between the coated and non-coated parts of an excellent quality (with a border of even less than 25 micrometers), and makes it possible to restore the original physical qualities of the substrate there where the coating has been removed.
According to the invention, said aim is reached by means of a process according to which, by means of a solid-state pulse laser or at least one pulsed laser diode, a pulsed laser beam whose wave length(s) is/are comprised between 10 micrometers and 360 nanometers is directed onto the coating to the spots where the latter has to be removed, whereby the applied wave lengths are such that the absorption of the substrate and of the coating as a whole is less than 30%.
It is already known to use a laser for the selective removal of layers in the coating process of the semiconductor industry, as well as for the removal of oxides from metallic surfaces.
In these cases, use is made of a gas laser. Such a type of laser cannot be used for transparent materials which, if their initial physical qualities are to be restored, do not tolerate that any compounds of the layers are diffused inside the material, and which only endure feeble increases in temperature.
U.S. Pat. No. 5,235,154 for example, concerns the production of integrated circuits and semiconductors, and describes a process which makes it possible to repair interruptions in circuits consisting of two metallic layers, applied on an insulating organic layer, for example made of polyamide, covering in turn the insulating substrate, made of glass for example. The parts to be replaced of the metallic layers are successively eliminated by means of evaporation with the help of a pulsed laser, after which the removed part is sealed again with a chemical coating in the shape of vapour. Only small surfaces are laser-treated, and the preservation of the optical qualities of the substrate cannot be guaranteed. Moreover, the whole is not transparent or translucent.
The process according to the invention makes it possible to preserve the optical qualities of the translucent or transparent substrate. Moreover, it can be applied to a substrate/coating as a whole which may have a light transmittance of more than 80%.
U.S. Pat. No. 5,958,268 describes a selective ablation process of unwanted materials, for example the ablation of oxides from a metal substrate, the ablation of silicon from a quartz substrate or the ablation of paint from a metal substrate. The ablation is obtained thanks to the combined effect of an inert gas, thrust onto the surface to be treated, and a laser beam of energetic photons.
It is necessary to use photons which are sufficiently energetic, i.e. with an energy comprised between 7 and 22 eV, which restricts the choice of the laser. Preference is given to a UV laser, for example a gas laser of the KrF type whose wave length is 248 nm and which allows for a photonic energy of 5 eV.
The above-described process does not allow for the selective removal of the coating from a transparent, relatively thick substrate made of glass or plastic, such as a wind screen, as it would result in the substrate being heated and the creation of micro-cracks, or as it would be inefficient due to the absorption of the energetic photons.
According to the invention, a solid-state laser having wave lengths between 10 micrometers and 360 nanometers is used, for example an Nd:YAG laser, whose photons only amount to 1.167 eV or 2.32 eV when being duplicated. The risk of damaging a chemical compound with this type of laser, producing photons which are little energetic, is very small.
DE-A-197 15 702 describes an ablation process of one or several layers by means of a self-regulating laser. According to this process, the ablation is brought about by means of a laser having such a wave length that the laser beam is absorbed by the coating to such an extent that the ablation level of the material of said coating is exceeded, whereas this level is not reached in the substrate. The optimal wave length is selected as a function of the absorption of the coating and of the substrate, whereby the relation of the absorption to the efficiency of the ablation is well known. In order to be able to better produce the optimal wave length, use is made of a laser with an optical parametric oscillator (OPO). As the alignment of the elements is very difficult and the operational stability requires an environment which is little aggressive, lasers with an OPO are practically useless outside the lab.
As the above-mentioned process is based on the absorption of the wavelength of the beam by the coating, the absorption of the wavelength of the beam must be significant and considerably different from that of the substrate.
According to the invention, the aforesaid is not required. The absorption of the coating with the used wave lengths may be feeble, even less than 10%, and the absorption of the transparent or translucent substrate may differ but only little from that of the coating. A mere observation of the spectrometric qualities of the coating and of the substrate will not be sufficient to meet the required efficiency and quality demands for, for example, the ablation of the layer or layers in order to reduce the thermal radiation emitted or transmitted by a wind screen, a car window or the window of a building.
According to the invention, a beam with a pulse lower than 30 ns and a wavelength which is preferably situated between 1,200 nanometers and 400 nanometers is preferably directed onto the coating.
The pulse laser beam can be sent through an optical imaging system and/or a diaphragm to improve the quality of the transition between the covered and noncovered surfaces.
The pulse laser beam is optimally directed onto the coating through the substrate.